Blocking a Multicast Network Device ? 239.255.255.250 / 224.0.0.251

ive got that media receiver here which works over multicast. and i wanna fully block it so that my pc isnt able to communicate with it and vice versa. i mean, i dont need the device when its about my pc and not tv. im speaking about these addresses :

239.255.255.250 multicast upnp

224.0.0.251 multicast dns

its the thing which receives data from the internet for the TV also called IPTV i believe.
it has a local network address ofc...

basically i just wanna know how can i completly block it ? its weird cause ive blocked the local ip address in the firewall and still it seems like theres a connection to it.

im on linux mint 15 if that is important.

im fiddling around with all this stuff for a long time by now without " real " success.
but i came to the conclusion that the problem is IP6 and Multicast related. pretty sure about that.

but as u can imagine, im no network expert so its kinda hard to figure it all out. and ive got no help at all. i mean im reaching a point where i dont see any wood cause of to many trees.